During the month of August 2002 researchers of the Athena project were the first in the world to successfully operate an instrument capable of producing many thousands of anti-hydrogen atoms each hour at a very low temperature. The Athena project is the fruit of a collaboration between the Italian national institute of nuclear physics (Infn) with other Italian universities and European, Brazilian and Japanese physics institutes. The atoms of anti-hydrogen produced consist in a particle called anti-electron, identical to the electron in all respects except it is positively charged, orbiting around a negative nucleus made by an anti-proton. The study, which provides an important leap forward in the field of antimatter studies, will be published by the journal Nature on September l9th. Being able to make low-temperature anti-hydrogen atoms is the first step towards obtaining a good number of anti-atoms stably confined in a small volume of space. When even this last objective will be reached, it will be possible to study anti-hydrogen in detail and confront it with hydrogen. The information thus obtained could help shed light on how the Universe dotted with matter in which we live first formed.

Everything we know today, from galaxies to the objects we touch every day, including our own bodies, is made of matter. That is to say, of atoms made of electrons, protons and neutrons. But theoretical studies tell us that in the first instants after the Big Bang an equal quantity of antimatter as of matter must have been formed. Antimatter is made of particles with the same mass as their matter counterparts but with an opposite charge (just like anti-electrons, identical to electrons except they are positively charged). The coexistence of these opposites lasted only a very short time: almost immediately, particles of matter started to collide with antimatter particles and annihilate, transforming into pure energy. The small percentage of the initial matter 'left over' after this process was nonetheless sufficient to form every body present in the known Universe.

Shut in their world of matter, for thousands of years not even the wildest dreamers have imagined the possibility of a world exactly like ours but reversed. This world might have disappeared completely a long time before the first stars were born or maybe part of it survived in some island inside our Universe. The idea of the existence of antimatter first
formed in the mind of a physicist who was not yet thirty years old, Maurice Dirac. In the following years proof of the existence of antimatter kept accumulating. Among others Carl David Anderson, Patrick Blackett and Giuseppe Occhialini discovered the anti-electron (also called the positron) in 1932. In 1955 Owen Chamberlain, Emilio Segrè, Clyde Wiegand and Tom Ypsilantis discovered the existence of the anti-proton, while in 1956 Bruce Cork, Glen Lambertson, Oreste Piccioni and William Wentzel identified the anti-neutron: a particle which, like the neutron, is only globally free from electrical charge. In 1965 a group led by Antonino Zichichi discovered the nucleus of anti-deuterium, made of an anti-proton and an anti-neutron: this was the first experimental evidence of the existence of antimatter in the strict sense, where anti-protons and anti-neutrons combine just like the common protons and neutrons do.

"Although it has been attracting scientists' attention for many decades, the elusive antimatter still has many mysteries to it. Many experiments are being carried out throughout the world in order to produce antiparticles in accelerators and to study their peculiarities. Many Infn physicists are involved in this kind of research. Scientists hope to understand why a portion of matter survived the remote destruction of matter and antimatter, since it appears that initially they were both present in equal quantities. The production of a good quantity of "cold" anti-hydrogen atoms carried out by the Athena project represents an important step forwards towards the study of the general properties of anti-hydrogen. These may then be compared to the properties of its common antagonist, hydrogen," in the words of Alberto Rotondi, Professor in physics at the university of Pavia and Infn associate.

The Athena experiment is carried out at the Cern laboratories of Geneva. The goal of the experiment is not only to produce atoms of anti-hydrogen, since these had already been made in small quantities in previous experiments. The aim is especially to obtain these atoms at a temperature nearing the absolute zero, since it is only in these conditions that in future it will be possible to close them in a trap of sorts, shut off by magnetic fields. This next step will be instrumental in allowing successive analysis.

In the Athena experiment antimatter particles, which are needed as “building blocks” to make the anti-atoms, are produced inside the accelerators. But in this way the energy, and thus the temperature, they possess, is extremely high. Therefore a drastic cooling is necessary. In the case of anti-protons the procedure is especially complex. The result is obtained using a special instrument called Antiproton Decelerator. Then their energy is reduced 10 billion times more, first by making them pass through sheets of a special material and then by "capturing" the anti-particles in a kind of electric and magnetic field trap. At the same time as the anti-protons, a cold and dense cloud of anti-electrons is prepared. Then the two clouds are brought into contact for about 200 seconds under high vacuum, where there is virtuaIly no matter Finally the formation of anti-hydrogen is verified with a special detector connected to a complex software system that analyzes the signals the detector emits.

"The over a hundred thousand anti-atoms made by Athena had a very brief life because in the experiment it was decided to make them collide with the walls in order to observe their annihilation. Confining techniques already known for ordinary atoms will soon be used to trap them inside a magnetic field box. At that point it will be possible to verify whether they interact with photons, the particles that make up light, in the same way as normal atoms. Then it will be very interesting to study the gravitational behavior of anti-atoms, in order to find out whether they "fall" to the ground following the same laws that apply to atoms. In a way, this is the modern version of the famous experiment carried out by Galileo from the leaning tower of Pisa", says Vittorio Lagomarsino, Professor of physics at the university of Genoa and Infn associate.